Castalius rosimon has white upperwings with black spots and blackish brown edges, the basal area of both wings is pale blue and shiny. The underside of the wings is white with black spots, black stripes and prominent black stripes on the sub-basalts of both wings. The black spots on the underside of the species vary depending on the season they live. The head, antennae, chest and abdomen are black, the antenna rod is surrounded by white, between the eyes and the back of the head are white. This species has a white tipped filamentous tail on the vein 2 of the hindwings.
Larva: larvae of Castalius rosimon have a broad, flattened body, bright green in color with a yellow stripe on the dorsal and yellow spots on the sides of the body.
Pupa: Castalius rosimon pupae are shiny black or green.